No substitute for primary forest

A little over five years ago, a controversial and spectacularly erroneous paper appeared in the tropical ecology journal Biotropica, the flagship journal of the Association for Tropical Biology and Conservation. Now, I’m normally a fan of Biotropica (I have both published there several times and acted as a Subject Editor for several years), but we couldn’t let that paper’s conclusions go unchallenged.

That paper was ‘The future of tropical forest species‘ by Joseph Wright and Helene Muller-Landau, which essentially concluded that the severe deforestation and degradation of tropical forests was not as big a deal as nearly all the rest of the conservation biology community had concluded (remind you of climate change at all?), and that regenerating, degraded and secondary forests would suffice to preserve the enormity and majority of dependent tropical biodiversity.

What rubbish.

Our response, and those of many others (including from Toby Gardner and colleagues and William Laurance), were fast and furious, essentially destroying the argument so utterly that I think most people merely moved on. We know for a fact that tropical biodiversity is waning rapidly, and in many parts of the world, it is absolutely [insert expletive here]. However, the argument has reared its ugly head again and again over the intervening years, so it’s high time we bury this particular nonsense once and for all.

In fact, a few anecdotes are worthy of mention here. Navjot once told me one story about the time when both he and Wright were invited to the same symposium around the time of the initial dust-up in Biotropica. Being Navjot, he tore off strips from Wright in public for his outrageous and unsubstantiated claims – something to which Wright didn’t take too kindly. On the way home, the two shared the same flight, and apparently Wright refused to acknowledge Navjot’s existence and only glared looks that could kill (hang on – maybe that had something to do with Navjot’s recent and untimely death? Who knows?). Similar public stoushes have been chronicled between Wright and Bill Laurance.

Back to the story. I recall a particular coffee discussion at the National University of Singapore between Navjot Sodhi (may his legacy endure), Barry Brook and me some time later where we planned the idea of a large meta-analysis to compare degraded and ‘primary’ (not overly disturbed) forests. The ideas were fairly fuzzy back then, but Navjot didn’t drop the ball for a moment. He immediately went out and got Tien Ming Lee and his new PhD student, Luke Gibson, to start compiling the necessary studies. It was a thankless job that took several years.

Using data from 138 studies from Asia, South America and Africa comprising 2220 pair-wise comparisons of biodiversity ‘values’ between forests that had undergone some sort of disturbance (everything from selective logging through to regenerating pasture) and adjacent primary forests, we can now hammer the final nails into the coffin containing the putrid remains of Wright and Muller-Landau’s assertion – there is no substitute for primary forest.

Our metrics were fairly straight forward – after standardising effect sizes (i.e., the difference between measured variables) for species richness, species abundance, community structure, forest structure and demographics, degraded forests had without fail lower biodiversity values (perhaps with the exception of demographics which are extremely difficult to measure precisely and accurately for sufficient samples).

Another interesting finding was that birds were by far the most sensitive taxon – once you start to degrade tropical forests, birds start to drop off the perch almost immediately (and yes, we did test for lag effects, before you ask). Arthropods and plants were the next-most sensitive, followed by mammals (there seemed to be in some cases and actual ‘positive’ response by mammals, driven largely by higher abundance of certain species [e.g., rats] in disturbed forests).

Another not-so-surprising finding was that Asia had the worst outcomes – biodiversity is failing faster there than anywhere else in the tropics. We also found, again rather unsurprisingly, that there was a gradient of responses, with the worst effects felt in abandon agricultural plots, and the least in selectively logged forests (although all forms of disturbance constituted some biodiversity loss).

Of course, it must be said that we live in a less-than-perfect world (really?), so even though this evidence is incontrovertible, it doesn’t mean that suddenly everyone will preserve all remaining primary tropical forest on the planet (wouldn’t that be lovely). So we need to consider the secondary and degraded forests in our conservation planning too – even bad forest is better than no forest at all.

However, we cannot dismiss the notion that there is no replacement for primary forests, that we should definitely not being staying stupid shit like ‘oil palm plantations hold just the same amount of biodiversity as primary forests’ [see previous post regarding that shining display of ignorance]. We need to conserve as much remaining primary forest as we can, while striving to regenerate those areas that have already come under the fire of development.

If you’d like a reprint, just email me here (or any other co-author, for that matter) and I’ll send you one.

24 responses

[…] forest specialists, and if this is a priority it is clear that primary forests must be protected (as previous papers have also suggested). However, it also appears that secondary forests have the potential to provide similar ecosystem […]

[…] there was relatively little biotic homogenisation. So while it is obvious from previous work that the maintenance of undisturbed forests is vital to conserve tropical forest biodiversity, it is also obvious that degraded forest can play an important role in conservation. This is […]

[…] An area bigger than the entire Indian landmass is now used for timber production in the tropics. This logging is largely selective and leads to degradation with loss of specialist species and ecosystem services like carbon storage. However, many have also argued that these forests should be considered a priority for protection since they are at danger of conversion to other land-uses such as agriculture. In addition the impact of logging on tropical forest biodiversity appears to be less negative than other human impacts. […]

[…] never achieve what existing forest already does. We know now from various parts of the world that biodiversity is nearly always much higher in primary forest, and that the carbon structure of the forest (especially below-ground carbon) can take centuries to […]

[…] never achieve what existing forest already does. We know now from various parts of the world that biodiversity is nearly always much higher in primary forest, and that the carbon structure of the forest (especially below-ground carbon) can take centuries to […]

[…] to my attention through various channels – not least of which was their citation of one of our previous papers ;-), as well as a blog post by Phil himself. I was so impressed with it that I made my first […]

[…] go further to ensure that lightly and moderately disturbed secondary forests are also protected. These forests are still very valuable both for carbon storage and for biodiversity. A carbon threshold should be established to ensure that land use change remains carbon neutral, or […]

[…] go further to ensure that lightly and moderately disturbed secondary forests are also protected. These forests are still very valuable both for carbon storage and for biodiversity. A carbon threshold should be established to ensure that land use change remains carbon neutral, or […]

[…] go further to ensure that lightly and moderately disturbed secondary forests are also protected. These forests are still very valuable both for carbon storage and for biodiversity. A carbon threshold should be established to ensure that land use change remains carbon neutral, or […]

[…] and the world. The idea is that because of intense habitat fragmentation, isolated patches of primary (or at least, reasonably intact secondary) forest can be linked by planting some sort of long […]

[…] lot of disparate studies collated to provide insight into broad-scale pattern (e.g., see our recent meta-analysis on the value of primary forests for tropical biodiversity published in Nature last […]

[…] recommendations are that existing tracts of native forest MUST be conserved first (given the irreplaceableness of primary forests), followed by a concerted effort to regenerate areas between existing native fragments to maximise […]

[…] forests, when you cut them down or degrade them, millions of other species go extinct (e.g., see my previous post covering our Nature paper). For the smaller seaweed species, the number and diversity of dependent invertebrates are massive, […]

Oh, and to the cretin that didn’t have the bollocks to sign his own name to the comment regarding Navjot’s cause of death, my commenting policy clearly states that you don’t get to remain anonymous. Also, if you don’t have a sense of humour, read something else. It was a joke. Get a life.

I’m familiar with these and some of the others as well – I use them in some graduate classes here, and students very much enjoy the back and forth. If memory serves D&W found that diversity was highest in old secondary forest that is near primary forest, but no matter what old growth forest was still the best, is that right? At any rate, my suggestion wasn’t that their assertions shouldn’t be tested, just that I’m not sure they were taking the position that disturbed landscapes were equivalent to old growth. Again, nice paper – congratulations to you all.

One minor quibble with your post above. I don’t actually think your study is a test of Wright and Muller-Landau’s argument at all because they never suggested that secondary forest and other disturbed areas were a “substitute” for primary forest. They argued that shifts in human demography would lead to less forest being cleared and cleared areas being left to regenerate, thereby allowing many species to persist that it had previously been suggested would be lost immediately.

In fact, they actually state that a major source of uncertainty in their analyses is that we lack a good understanding of how well disturbed landscapes support biodiversity and that “[This] source of uncertainty cannot be resolved until the proportion of tropical species that require old growth forests is known (p. 298). Essentially, they are calling for exactly the type of study you all did!

Gibson et al. provides a very nice test of a key assumption of their argument, but not of their argument per se. I found many aspects of Joe and Helena’s argument flawed, but let’s at least do them the courtesy of representing it accurately.

Thanks, Emilio. We dealt with the other components (which were equally erroneous) about which you speak (e.g., human demography) in our 2006 Biotropica paper, but I have to point out that Joe and Daisy Dent went on to claim very high biodiversity in secondary forests in their 2009 review. Therefore, I think I’m more than justified in attacking him on this issue.